I am using Ubuntu 12.04 and I want to protect some pictures of another user using the su command (I have admin privileges). How can I protect a file using the terminal while still allowing the rest of the users to read it?

I recommend to use Truecrypt to create an encrypted "removable disk" to 1 file.
If you create for example 1GB Truecrypt partition and you can protect this file with password. (Expert option: you can set 2 password for that file: with first password you see a normal part of file and with password 2 you see the hidden part)
You can mount this file in Linux, Mac or in Windows (mount to drive e:) and you will show that 1GB file as like a pendrive/flash drive, and you can unmount when you want.

If you create that 1GB file (or any size, or a full disk or pendrive) you can add any extension for that file to hide. (Never place that file at your home directory as secret.truecrypt.file.with.pwd lol)

If you use any system file name in system folder maybe normal user never notice it's not a system file it's a private encrypted file.